Gravure is one of the principal four traditional printing processes (the others being offset lithography, flexography and screen printing). Each of these processes is distinguished from the others by where the ink resides in relationship to the surface of the master and which areas of the master provide the non-ink or background areas. Flexographic plates have a raised surface that accepts the ink, the background being the recessed surface. Offset lithography has the ink and the background coplanar, with the difference between ink and background areas being determined by surface chemistry. Screen printing has the ink printing through holes in the master, with the background being provided by the remaining master surface. Gravure has the ink residing in indented cells, background being provided by the remaining upper surface.
Each printing method demands its own types of ink, its own imaging system(s) and its own presses. Each process has its own advantages and disadvantages.
“Gravure Process and Technology” from the Gravure Association Of America (page 380) explains the advantages and disadvantages of gravure. Gravure is regarded as a very simple process compared to flexo and offset lithography. It is more adaptable to less expensive paper, and it gives better image quality and color consistency. Its main disadvantage is the high cost and the time needed to engrave gravure cylinders. This makes the gravure process inappropriate for short runs and indeed it finds its place in very long runs of up to and beyond a million impressions.
Gravure cylinders are prepared by either imaging a photoresist through a film and then chemically etching the metallic surface of the cylinder, or by directly engraving the cylinder with some type of engraving tool. Electromechanical engraving is a slow process. Etching has to be very carefully controlled as it tends to spread laterally as it progresses downwards to give undercutting of cell walls.
In recent years, with the advent of computers, origination for reproduction by printing processes has become available in digital form and much work has been done in imaging printing plates digitally and more specifically using a modulated laser beam for such imaging. Because of the necessity for engraving specific holes to produce the cells needed for gravure, gravure printing has a long history of attempts to use lasers for digital imaging. Thus U.S. Pat. No. 3,636,251 to Daly et al describes a system for engraving intaglio printing plates by forming cells in a metal plate using a pulsed output laser. UK Patent Application, GB 2034636A claims that the former patent method has the disadvantage that it tends to produce rims round the gravure cells. The British patent claims an advantage in using polymeric printing blanks for laser engraving, where such blanks have high thermal conductivity. The areas struck by the laser are vaporized. Carbon black may be incorporated into the polymer to improve absorption of the laser energy. More recently, U.S. Pat. No. 5,126,531 to Majima et al described a method of producing a gravure printing plate using a thermoplastic resin sheet containing about 20 percent of carbon. The plate was wrapped around a cylinder and imaged by a semi-conductor laser beam.
U.S. Pat. No. 6,048,446 to Michaelis suggests building up walls by plating using a photoresist mask, but such masks are of thicknesses down to 1 micron, which makes them suitable for IR imaging but makes it impossible to then build up walls with straight sides to a thickness of 12 or more needed for good quality gravure cells. Thick plating using thin masks tend to spread so that they overhang the thin mask—a problem that the '446 patent fails to address.